Molybdenum base alloys



United States Patent MOLYBDENUM BASE ALLOYS Dean D. Lawthers, Verona,Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh,Pa., a corporation of Pennsylvania No Drawing. Application July 30, 1956Serial No. 600,701

6 Claims. (Cl. 75-211) This invention relates to molybdenum base alloysprepared by pressing and sintering powder mixtures of molybdenum andsuitable alloying constituents, and to a method of preparing suchalloys.

Molybdenum base alloys containing such alloying constituents as thetransition elements are known and have been prepared by arc-casting amixture of the components that have been pressed to the form of aconsumable electrode. Molybdenum base alloys prepared by such methodsare not homogeneous, and have coarse elongated grain structures.Furthermore, impurities in solution in arc-cast molybdenum base alloysprecipitate at the grain boundaries. These factors make it verydiflicult to work arc-cast molybdenum base alloys by conventionaltechniques of rolling, forging and swaging, and the recovery of materialfrom working arc-cast molybdenum alloys is exceedingly low, frequentlybeing less than 50 percent.

It is among the objects of the present invention to provide molybdenumbase alloys that are homogeneous; that are characterized by a fine grainstructure; that have high strength at elevated temperatures; that may beworked by conventional techniques; and that can be worked atsatisfactory material recovery.

It is another object of the invention to provide a method for producingalloys characterized as in the foregoing object.

In accordance with the invention, molybdenum base alloys containing asalloying components one or more of zirconium, niobium, titanium,tantalum and vanadium are prepared by powder metallurgical techniques ofpressing and sintering under conditions described below. The resultantalloys have an equi-axed fine grain structure, a low oxygen content onthe order of below about 0.01 percent and are carbon-free except fortraces that may be present as an impurity. The resulting alloys,contrary to the characteristics of arc-cast molybdenum base alloys ofrequiring very high temperatures of the order of 1315 C. or more forworking, or the use of complex and tedious techniques to reduce thestructure to workable form, may be'readily worked as prepared byconventional techniques, for the resultant alloys are homogeneous andfine grained and the oxygen, or other impurities, that may be presentdoes not segregate at the grain boundaries. Furthermore, the materialrecovery upon working alloys of this invention exceeds SO percent andfrequently exceeds 90 percent, a singular improvement as compared to therecovery of 50 percent or less of the arc-cast alloys.

Alloys of this invention contain one or more of the alloyingconstituents in an amount within the range of solubility of the alloyingconstituent in molybdenum, for alloys that contain the alloyingconstituents in excess of-its solubility are brittle and, therefore, maynot be worked. Typical compositions contain, by weight, 0.1 to 2 percentof zirconium, or 0.1 to 10 percent of any of titanium, tantalum,vanadium, and niobium with the balance being essentially puremolybdenum. Other elements and impurities may be present as long as theydo not deleteriously affect the resultant alloys.

The molybdenum base alloys are produced by powder metallurgicaltechniques. In a typical procedure a commercial molybdenum powder,usually of average particle size that will pass a minus 325 mesh screen,is treated to eliminate, essentially, the oxygen content of themolybdenum, for example by hydrogen reduction in a furnace at a suitablyelevated temperature so that the reaction can proceed and the oxygen canbe removed. Heating in hydrogen for about 1 to 6 hours at a temperatureof about 500 to 1000 C. will normally be satisfactory, with the optimumconditons being determined primarily by the amount and nature of theoxygen content of the molybdenum. The resultant oxygen-free molybdenumpowder is then mixed with the alloying constituent in a suitableblender. When the powders are mixed, they are pressed at a pressure ofthe order of 10,000 p.s.i. or higher to compact the powders sufiicientlyto permit further handling. The pressed article is then sintered at atemperature of at least 1700 C., preferably above 2000 C., but below themelting point of the alloy; about 2 to 30 or more hours at sinteringtemperature is sufificient to insure satisfactory alloying. The densityof the resultant alloy must be at least percent of theoretical to insuregood workability. During sintering, uniform diffusion of the alloyingconstituent through the molybdenum occurs and a true alloy ingotresults.

Handling, storage and processing of the molybdenum powder until it hasbeen pressed to a compact most suitably is accomplished under inertconditions, for the rate of oxygen pick-up of molybdenum powder is quiterapid and the presence of significant quantities of oxygen in theresultant alloys adversely afiects their properties. An atmosphere ofhelium or argon, and the use of a vacuum have been found to besatisfactory. While the presence of oxygen should be minimized, I havefound that it is not essential to exclude it entirely, for it can betolerated for up to about five minutes without deleterious oxygenpick-up occurring. For analogous reasons, the sintering of the compactis accomplished under a non-reactive atmosphere. A vacuum of the orderof less than 0.1 micron is satisfactory. An inert gaseous atmosphere canalso be used; however, it is ditficult to obtain an inert gas of apurity sufiicient to avoid oxygen pick-up at the conditions oftemperature and time that are used in the sintering step.

Alloys prepared as just described, in addition to being workable byconventional practices, are more uniform than arc-cast alloys for thepowder mixture can be homogenized prior to pressing and sintering, andnormal and inverse segregation during solidification does not occur forthe metal is never molten. Uniformity also is promoted by sintering thecompositions for several hours to permit substantially completediffusion of the components. The small grain structure of the alloysindicates the presence of a large grain boundary area which, togetherwith the absence of grain boundary segregation, contributes to theirworking characteristics.

Several alloys were prepared and tested for their high temperaturestrength. During processing contact between oxygen and the oxygen-freemolybdenum powder was entirely excluded by blending and pressing thepowder mixtures in a helium atmosphere and conducting the sintering stepin a vacuum. Stress relieved samples of the various alloys were thentested for their stress characteristics at high temperature. In thefollowing table are the data obtained; the amount of alloyingconstituent in the alloys is by weight percent.

Table Hours at Stress of Molybdenum Base Alloys at 982 0. Stress, p.s.i.

1.89 1.75 1.457 1.647 v. Nb? T1. Ia-.

All of'the data on the zirconium alloy were obtained on a-single sample,it being used at the successively higher stresses until it finallyruptured after 23 hours at 100,000 p.s.i.

These data demonstrate the unusual strength of alloys prepared accordingto the method of the present invention. The extraordinary high strengthof the zirconium alloys is especially apparent. Indeed, these zirconiumalloys have the highest strength at elevated temperatures of any alloysof which I am aware. This unusual aspect of these alloys was furtherdemonstrated by testing an 0.4 percent zirconium-molybdenum alloy thatwas prepared by the described process at 40,000 p.s.i. at 982 C. Duringthe test the furnace overheated and the sample was at 40,000 p.s.i. for12 hours at a temperature of 1093 C. This sample ruptured after 170hours. These data and that included in the table demonstrate thesingular strength of this alloy, especially when it is considered thatunalloyed molybdenum that is stress relieved ruptures in about 3.6'hoursat 982 C. at a stress of only 18,000 p.s.i.

A molybdenum base alloy of this invention containing 1.89 percentvanadium has a 100 hour rupture strength of 17,000 p.s.i. at 982 C., andthis, too, is a significant improvement over unalloyed molybdenum.Moreover, this alloy may be cold worked at a 90 percent reduction inarea with a material recovery of over 90 percent. When a MoV alloy ofthe same vanadium content is prepared by arc-casting, working results ina recovery of less than 50 percent.

Similarly favorable comparisons occur upon considering the other alloysof my invention. Thus, a 1.75 percent niobium alloy in the stressrelieved condition has a 100 hour rupture strength of 35,000 p.s.i. at982 C.; a one percent niobium alloy was tested at 35,000 p.s.i. at 982C. and did not rupture after 500 hours, the test being stopped duetofurnace failure. A stress relieved 1.45

percent titanium alloy has a 100 hour rupture strength of over 30,000p.s.i. at 982 C., as does a stress relieved 1.64 percent tantalum alloy.It is thus apparent that all of the alloys of this invention havesignificant high temperature strength.

A particular advantage of these alloys as compared with pressed andsintered molybdenum is that the alloys have the higher recrystallizationtemperature, generally ranging from 100 to 300 C. higher than that ofthe molybdenum. Furthermore, the hardness of the alloys exceeds that ofmolybdenum at 1100 C., in some instances by as tion temperature of -170C. in the cold worked condition and 40 C. in the recrystallizedcondition, the corresponding temperatures of an 0.1 percentzirconiummolybdenum alloy of this invention are 160 C. and 70 C.,respectively. The other alloys of the invention, similarly to unalloyedmolybdenum, evidence sufiicient ductility at about room temperature topermit conventional handling.

Bars or slabs of the sintered alloys can be readily hot worked atmoderate temperatures far below the temperatures required to workarc-cast ingots. Thus, sintered bars wereprepared from each of thealloys in the table, the bars being 7 inches long by 1.5 inches wide andabout 0.6 inch thick. The sintered bars were heated to temperatures offrom 871 C. to 1204 C., in a non-oxidizing atmosphere, were hot rolled,with intermediate reheating if necessary, to strips of a thickness of0.06 inch. Reductions of to 95% were etfected. The sintered bars can bereadily forged, swaged or extruded. A recovery of usable strip of over50% was realized in all instances, and in some cases the recovery ofusable rolled metal was in excess of of the sintered bar. It will beappreciated that the high recovery and low hot working temperatures areoutstanding benefits of the present invention.

The hot worked strip can be cold rolled or cold worked after reductionto a thickness of between about 0.05 and 0.06 inch. The hot working canbe effected to produce strips, bars, rods, wire and the like. The shapedmembers produced by hot or cold working can be stress-relieved byannealing, at for example, 982 C. for one hour while in a non-oxidizingatmosphere.

It will be understood that ternary and other molybdenum base alloys maybe prepared from two or more of the alloying components using thesintering and working procedures of the present invention. An'excellentalloy comprises 2% niobium, 3% titanium and thebalance molybdenum.Another ternary alloy comprises 0.5% zirconium, 1% niobium and thebalance moylbdenum. A quaternary alloy comprises 2.5% titanium, 2%niobium, 0.4% zirconium and the balance molyb-. denum. v I

It will be understood that the alloys may be preparedv by simplyadmixing the molybdenum powder and the alloying component in powderedform if a highly ,de-'

oxidized molybdenum having less than 0.01% of oxygen is employed, andthe alloying powder is similarly free from oxygen.

According to the provisions of the patent statutes, I

have explained the principle of my invention and have described what Inow believe to be its best embodiment. However, I desire to have itunderstood that the invention may be practiced otherwise than asspecifically described.

I claim:

1. In the process of preparing members from molybdenum base alloys, thesteps comprising intimately admixing molybdenum powder having an oxygencontent of below 0.01%. by weight with at least onesubstantiallyf oxygenfree powdered metal in the given proportions se-' lected from the groupconsisting of from 0.1 to 2% by weight of zirconium, from 0.1 to 10% byweight of niobium, 0.1% to 10% by weight of tantalum, from 0.1 to 10% byweight of titanium and from 0.1 to 10% by weight of vanadium,compressing the resultant powdered mixture to a compact, and sinteringthe compact in the absence of a reactive atmosphere at a temperatureabove about 1700 C, but below the melting point of the mixture for aperiod of time to alloy the powders and to produce a sintered memberhaving a density at least.

% of the theoretical density of the alloy, said powders being maintainedsubstantially free from contact with reactive gases.

2. The process of claim 1, wherein the sintered member is heated to atemperature of from about 871 C- to 1204 C. in a non-oxidizingatmosphere and hot worked asaaaae to desired shape, the resultant usableworked alloy mem ber comprising over 50% of the sintered member.

3. That method of preparing a molybdenum base alloy that, as prepared,can be worked at a material yield in excess of 50 percent, that consistsof deoxidizing a molybdenum powder with hydrogen at an elevatedtemperature, blending with the resultant reduced molybdenum powder amember selected from the group of powders consisting of 0.1 to 2 percentof zirconium, 0.1 to percent of vanadium, 0.1 to 10 percent of niobium,0.1 to 10 percent of titanium, and 0.1 to 10 percent of tantalum,pressing the resultant powder mixture to a compact, and sintering thecompact in a non-reactive atmosphere at a temperature below the meltingpoint of the resultant alloy to alloy the powders and result in an alloyhaving a density of at least 95 percent of theoretical, said powdersbeing maintained in an inert atmosphere prior to pressing for at leastall the period of time involved that is in excess of five minutes.

4. Method according to claim 3, said sintering being conducted in avacuum of below about 0.1 micron pressure.

5. A molybdenum base alloy that, as prepared, can be worked at amaterial yield in excess of 50 percent, that consists of the alloyprepared by deoxidizing a molybdenum powder with hydrogen at an elevatedtemperature to an oxygen content of below 0.01% by weight, blending withthe resultant reduced moylbdenum powder a member selected from the groupof powders consisting of 0.1 to 2 percent of zirconium, 0.1 to 10percent of vanadium, 0.1 to 10 percent of niobium, 0.1 to 10 percent oftitanium and 0.1 to 10 percent of tantalum, pressing the resultantpowder mixture to a compact, and

sintering the resultant compact in a non-reactive atmosphere at atemperature below the melting point of the resultant alloy to alloy thepowders and result in an alloy having a density of at least 95 percentof theoretical, said powders being maintained in an inert atmosphereprior to pressing for at least all the period of time involved that isin excess of five minutes.

6. A molybdenum base alloy that, as prepared, can be worked at amaterial yield in excess of percent, that consists of the alloy preparedby deoxidizing a molybdenum powder with hydrogen at an elevatedtemperature to an oxygen content of below 0.01% by weight, blending 0.1to 2 percent of zirconium powder with the resultant reduced molybdenumpowder, pressing the blend of powders to a compact, and sintering theresultant compact in a non-reactive atmosphere at a temperature belowthe melting point of the resultant alloy to alloy the zirconium andmolybdenum and result in an alloy having a density of at least percentof theoretical, said powders being maintained in an inert atmosphereprior to pressing for at least all the period of time involved that isin excess of five minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,491,866 Kurtz et al. Dec. 20, 1949 2,692,216 Baker Oct. 19, 19542,776,887 Kelly et al. Jan. 8, 1957 OTHER REFERENCES Pipitz et al.:Powder Metallurgy Bulletin, vol. 7, No. 2 (August 1955), pp. 53-59.

1. IN THE PROCESS OF PREPARING MEMBERS FROM MOLYBDENUM BASE ALLOYS, THESTEPS COMPRISING INTIMATELY ADMIXING MOLYBDENUM POWDER HAVING AN OXYGENCONTENT OF BELOW 0.01% BY WEIGHT WITH AT LEAST ONE SUBSTANTIALLY OXYGENFREE POWDERED METAL IN THE GIVEN PROPORTIONS SELECTED FROM THE GROUPCONSISTING OF FROM 0.1 TO 2% BY WEIGHT OF ZIRCONIUM, FROM 0.1 TO 10% BYWEIGHT OF NIOBIUM, 0.1% TO 10% BY WEIGHT OF TANTALUM, FROM 0.1 TO 10% BYWEIGHT OF TITANIUM AND FROM 0.1 TO 10% BY WEIGHT OF VANADIUM,COMPRESSING THE RESULTANT POWDERED MIXTURE TO A COMPACT, AND SINTERINGTHE COMPACT IN THE ABSENCE OF A REACTIVE ATMOSPHERE AT A TEMPERATUREABOVE ABOUT 1700*C. BUT BELOW THE MELTING POINT OF THE MIXTURE FOR APERIOD OF TIME TO ALLOY THE POWDERS AND TO PRODUCE A SINTERED MEMBERHAVING A DENSITY AT LEAST 95% OF THE THEORETICAL DENSITY OF THE ALLOY,SAID POWDERS BEING MAINTAINED SUBSTANTIALLY FREE FROM CONTACT WITHREACTIVE GASES.